The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
One engine system being developed for controlled auto-ignition combustion operation comprises an internal combustion engine designed to operate under an Otto cycle. The engine, equipped with direct in-cylinder fuel-injection, operates in a controlled auto-ignition mode under specific engine operating conditions to achieve improved engine fuel efficiency. A spark ignition system is employed to supplement the auto-ignition combustion process during specific operating conditions. Such engines are referred to as Homogeneous Charge Compression Ignition (hereinafter ‘HCCI’) engines.
An HCCI engine operating in HCCI combustion mode creates a charge mixture of combusted gases, air, and fuel in a combustion chamber, and auto-ignition is initiated simultaneously from many ignition sites within the charge mixture during a compression stroke, resulting in stable power output, high thermal efficiency and low emissions. The combustion is highly diluted and uniformly distributed throughout the charge mixture, resulting in low burnt gas temperature and NOx emissions typically substantially lower than NOx emissions of either a traditional spark ignition engine, or a traditional diesel engine.
A typical HCCI engine is distinguishable from a conventional spark-ignition engine in that ignition of the charge mixture is caused by compression of the charge mixture under specific engine operating conditions. The typical HCCI engine transitions between HCCI combustion mode and spark-ignition combustion mode, depending upon predetermined operating conditions.
Applicants have successfully demonstrated smooth transition control between HCCI and SI/NTLC (SI with Non-Throttled Load Control) combustion modes utilizing dual independent, 2-step, cam profile switching mechanisms. In particular, transition from HCCI to SI/NTLC operation may be realized by cam phasing of low lift intake cams with Early Intake Valve Closing (EIVC) in conjunction with profile switching from low lift HCCI to high lift SI exhaust cam profiles. SI/NTLC engine operation is also possible using cam phasing of high lift SI intake cam with Late Intake Valve Closing (LIVC) if simultaneous switching of both intake and exhaust cam profiles between low lift HCCI and high lift SI cams is mandated.
However, depending on the lift and duration used for both the HCCI and SI cams and cam phaser authority and slew speed, smooth transition between HCCI and SI/NTLC combustion mode may be impossible beyond certain engine speed. In particular, a gap exists between the highest load reachable with HCCI operation and the lowest load reachable with SI/NTLC operation above a certain engine speed. FIG. 2, for example, illustrates an engine speed limit of substantially 2000 rpm if 4 mm peak lift and 120 degree duration intake and exhaust HCCI valve lift profiles and typical 10 mm peak lift and 240-260 degree duration intake and exhaust SI valve lift profiles are used, respectively. The highest load reachable with HCCI operation is limited by ringing (ringing limit=3 MW/m2) while the lowest load reachable with SI/NTLC operation is limited by combustion stability (COV of IMEP=3%). The HCCI ringing limit results from an inability to induct sufficient charge dilution with the prescribed low lift cams. The SI combustion stability limit results from excessive charge dilution with the prescribed high lift cams. Applicants have successfully demonstrated that selective cylinder deactivation by fuel cutoff may be employed to extend the engine low load operating limit in SI combustion mode by allowing stable operation of the active cylinders. Alternatively, it is believed that more complex cam profile switching mechanisms (e.g. three-step cam profiles) or continuously variable valvetrains could successfully be employed to extend both the high load HCCI and low load SI operating limits and close the gap between HCCI and SI operations (e.g. through intermediate lift and duration). However, an alternative utilizing the less complex 2-step, cam profile switching mechanisms and without additional cylinder deactivation hardware is desirable.